There is an increasing amount of pollution in the air we breathe in the work environment, homes, hospitals, public buildings, and vehicles, caused by a variety of toxic, volatile, and often malodorous and irritating chemicals. Also included in this pollution are potentially dangerous microorganisms such as viruses, bacteria, molds, yeasts, spores, and other pathogens.
In industry, toxic, hazardous, or contaminating chemical vapors, such as formaldehyde, benzene, chloroform, etc., have come under strict government regulation. However, there is often an accumulation of these airborne pollutants in the vicinity of chemical plants, food processing operations, sewage treatment plants, and utilities. Significant or even larger contaminant or pollutant levels can occur indoors because of tightly enclosed more energy efficient buildings.
Additionally, there is now the threat of terrorism in which dangerous chemicals or microorganisms are introduced into the atmosphere.
Six criteria pollutants were originally named by the Environmental Protection Agency, which is required to summarize published information on each of these pollutants, the documents referred to as criteria documents. These pollutants are sulfur dioxide, carbon monoxide, nitrogen dioxide, ozone, suspended particulates and volatile organic compounds. These pollutants are ubiquitous, and there is substantial evidence linking them to health effects at high concentrations. Three of them, ozone, sulfur dioxide and nitrogen dioxide, are also considered phytotoxins, and these substances are fairly easy to measure.
A variety of methods are currently used for removing pollutants from air, including adsorption e.g., in activated carbons, absorption in solution, filtration, coagulation, electrostatic precipitation, incineration, chemical reaction, condensation, etc. However, many of these methods cannot readily remove very tiny particles, such as gas molecules.
Absorption of toxic gases by water or aqueous solutions, such as HCl, NH3, HF, Cl2, H2S, amines, and the like, can be very effective. Likewise, many of these gases can be removed effectively by adsorption onto solid surfaces, particularly activated carbons. Absorption, adsorption, filtration, coagulation and electrostatic precipitation are physical methods of decontamination. Physical methods are of special interest where recovery of chemicals is economically desirable.
Chemical methods of decontamination include reactions of toxic or hazardous substances with oxidizing agents such as chlorine, chlorine dioxide, hypochlorite, ozone, peroxide, or reducing agents, such as dithionite, noble metal catalysts with hydrogen, metallic sodium, etc.
Both physical and chemical methods have drawbacks. For example, in a situation in which it is not desirable to recover airborne chemicals for reuse, the problems of disposal still exist. Additionally, physical techniques may suffer from saturation of the agent used to remove the pollutant. Chemical methods often require the use of reagents which are themselves quite toxic and often have problems of byproduct disposal. Other methods, such as incineration, can be uneconomical or even illegal in certain areas.
Perez de la Garza, in U.S. Pat. No. 5,227,144, discloses a process for decontaminating air to eliminate toxic gases. Air is circulated in a soaking chamber through air-permeable barriers that are continuously bathed in solutions containing chemical reagents which remove toxic, solid and gaseous pollutants from the air. The first barrier contains water to eliminate all solid particles suspended in the gas flow and to begin eliminating carbon monoxide. The second barrier contains an alkali metal hydroxide to eliminate nitrogen dioxide, hydrogen sulfide and prussic acid, as well as some of the ozone. The third barrier contains sulfuric acid to eliminate carbon monoxide. The fourth barrier contains an alkali earth hydroxide to eliminate carbon dioxide. The fifth barrier contains sodium nitrite to eliminate ozone. The sixth barrier contains potassium permanganate to eliminate sulfur dioxide. The seventh barrier contains a diluted solution of sodium hypochlorite to eliminate all vestiges of any of the reagents.
Kirts, in H1189, a Statutory Invention Registration, discloses a method for destroying chemical and biological warfare agents by heating contaminated air in a compressor by adiabatic compression, flowing the compressed hot air through a reaction vessel to provide sufficient contact time to kill chemical and biological warfare agents, partially cooling the hot compressed air in an aftercooler, and finally cooling the hot compressed air by expansion in a turbine.
Weinberg et al., in U.S. Pat. No. 5,009,869, disclose a method for treating air contaminated with chemical and biological pollutants in a closed system which includes an electrochemical reactor to regenerate the reagents. Contaminated air is treated in a wet scrubber zone containing a high surface area packing which is inert to the circulating aqueous scrubber liquid-electrolyte. The scrubber liquid-electrolyte may be circulated in a loop forming a closed system from the anode compartment of a cell to the wet scrubber zone wherein the polluted air is cleaned and the chemical and biological contaminates dissolved into liquid where they may be further broken down to products of substantially less hazard or toxicity. The scrubber liquid-electrolyte contains at least one electrochemically regeneratable degradant, such as a redox couple which in their active form chemically degrade, destroys, or disinfects the pollutant-containing scrubber-electrolyte. Ions are provided in the scrubber liquid-electrolyte to provide for ionic conductivity in the electrochemical cell. This system can be adapted to heating and cooling systems of buildings.
Zamansky et al., in U.S. Pat. No. 5,670,122, disclose a method for removing air pollutants from combustion gases by treating the gases with hydrogen peroxide or a mixture of hydrogen peroxide and methanol to remove one or more of nitric oxide, sulfur trioxide, light hydrocarbons, carbon monoxide, and trace amounts of mercury from combustion flue gas streams.